1. Field of the Invention
The invention relates generally to electrostatic chucks for holding a workpiece and, more specifically, to methods and apparatus for releasing a workpiece (such as a semiconductor wafer) from an electrostatic chuck.
2. Description of the Prior Art
Electrostatic chucks are used for holding a workpiece in various applications ranging from holding a sheet of paper in a computer graphics plotter to holding a semiconductor wafer within a semiconductor fabrication process chamber. Although electrostatic chucks vary in design, they all are based on the principle of applying a voltage to one or more electrodes in the chuck so as to induce opposite polarity charges in the workpiece and electrode(s), respectively. The electrostatic attractive force between the opposite charges presses the workpiece against the chuck, thereby retaining the workpiece.
A problem with electrostatic chucks is the difficulty of removing the electric charge from the workpiece and the chuck when it is desired to release the workpiece from the chuck. One conventional solution is to connect both the electrode and the workpiece to ground to drain the charge. Another conventional solution which purportedly removes the charge more quickly is to reverse the polarity of the DC voltage applied to the electrodes. This is described in the context of a chuck having two electrodes (a bipolar chuck) in U.S. Pat. No. 5,117,121 to Watanabe et al.
A shortcoming we have observed with these conventional approaches to removing the electric charge is that they fail to completely remove the charge, so that some electrostatic attractive force remains between the workpiece and the chuck. This residual electrostatic force necessitates the use of a large mechanical force to separate the workpiece from the chuck. When the workpiece is a semiconductor wafer, the force required for removal sometimes cracks or otherwise damages the wafer. Even when the wafer is not damaged, the difficulty of mechanically overcoming the residual electrostatic force sometimes causes the wafer to pop off the chuck unpredictably into a position from which it is difficult to retrieve by a conventional wafer transport robot.